Nature of the Dye Employed
Dyes are chemically of two different types, acid and basic; so-called acid dyes are the alkali salts of organic acids, while basic dyes are the chlorides, sulphates, etc., of organic bases.
For the tinting of film only “acid” dyes have been considered since it is not possible to make a complete selection from basic dyes alone, which would necessitate the use of acid and basic dyes in admixture—a procedure highly undesirable and in many cases impossible.
Any dye suitable for admixture with others to produce intermediate tints should possess the following properties:
A. The dye should be inert and not attack the gelatine or support. This is of fundamental importance as the gelatine coating of dyed film in many cases has a tendency to lose its flexibility, giving rise to what is known in the trade as “brittleness.”
Several dyes when employed at a concentration of 1% attack gelatine readily at 70 degrees F. and vigorously at 80 degrees F., especially in presence of small amounts of acid, producing a marked softening and often partial dissolution of the film. The effect is roughly proportional to the concentration of the dye and to the temperature, and varies with each individual dye.
Experience has shown that the gelatine coating of film which has been softened in this way by the dye becomes “brittle” on subsequent projection.
The actual factors in the production of brittleness are:
1. The hydrolysing action of acid which in many cases is added to assist dyeing. In case a solid acid is employed the heat encountered during projection will greatly accelerate this hydrolysis.
2. The corrosive action of the dye itself. Dyes vary considerably in this respect according to their particular composition. So far, it has not been possible to make any general classification of dyes in this connection, though nitro compounds appear to be particularly corrosive in their action.